A magnetic disk drive is a digital data storage device that stores digital data on a magnetic recording medium known as a disk. A disk, in general, contains tracks for storing the digital data. The tracks are defined by a servo pattern recorded in the magnetic layer, which itself is comprised of individual magnetic grains. Data is stored on the tracks of the disk as magnetic polarity transitions induced in these magnetic grains.
Increases in areal density of magnetic disks have been achieved through the use of smaller magnetic grains. The superparamagnetic effect, however, limits growth in areal density. That is, the smaller the grains become, the less thermally stable they become until, at some point, the grains are no longer stable at room temperature. This instability compromises the integrity of data on a disk.